Problem: Omar is 18 years older than Ashley. Fifteen years ago, Omar was 3 times as old as Ashley. How old is Ashley now?
Solution: We can use the given information to write down two equations that describe the ages of Omar and Ashley. Let Omar's current age be $o$ and Ashley's current age be $a$ The information in the first sentence can be expressed in the following equation: $o = a + 18$ Fifteen years ago, Omar was $o - 15$ years old, and Ashley was $a - 15$ years old. The information in the second sentence can be expressed in the following equation: $o - 15 = 3(a - 15)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $a$ , it might be easiest to use our first equation for $o$ and substitute it into our second equation. Our first equation is: $o = a + 18$ . Substituting this into our second equation, we get the equation: $(a + 18)$ $-$ $15 = 3(a - 15)$ which combines the information about $a$ from both of our original equations. Simplifying both sides of this equation, we get: $a + 3 = 3 a - 45$ Solving for $a$ , we get: $2 a = 48$ $a = 24$.